1. Field of the Invention
The art disclosed by the present specification relates to a semiconductor device having a semiconductor substrate on which a diode and an IGBT are formed.
2. Description of Related Art
In Japanese Patent Application Publication No. 2009-272550 (JP-2009-272550 A), there is disclosed a semiconductor device having a semiconductor substrate on which a diode and an IGBT are formed. In general, such a semiconductor device is called an RC-IGBT. In this RC-IGBT, a cathode region of the diode and a collector region of the IGBT are formed in a range exposed to a lower face of the semiconductor substrate. Besides, a common electrode that conducts to the cathode region and the collector region are formed on the lower face of the semiconductor substrate.
In a conventional RC-IGBT having a common electrode that conducts to a cathode region and a collector region as in Japanese Patent Application Publication No. 2009-272550 (JP-2009-272550 A), a metal having a relatively large work function, such as AlSi or the like, is used for the common electrode. The metal having the large work function has a low barrier for a p-type collector region. Accordingly, the common electrode suitably conducts to the collector region. On the other hand, the metal having the large work function has a high barrier for an n-type cathode region. Thus, in the conventional RC-IGBT, with a view to making the common electrode conduct to the cathode region, an extremely high concentration of an n-type impurity is infused into the cathode region. When the high concentration of the n-type impurity is thus infused into the semiconductor substrate, amorphized defects are formed in the semiconductor substrate. Such defects are unfavorable because they cause the leakage of electric current. By laser-annealing the semiconductor substrate, such defects can be removed to a certain degree. However, it is difficult to completely remove the defects. In particular, in a region where the concentration of the n-type impurity is high, laser light is likely to be absorbed. Thus, laser light does not reach a deep position of the semiconductor substrate, and it is extremely difficult to remove defects that are present at a deep position.